Separation of t-Amyl alcohol from isobutanol by extractive distillation

ABSTRACT

t-Amyl alcohol and isobutanol cannot be separated from each other by distillation because of the proximity of their boiling points. t-Amyl alcohol can be readily separated form isobutanol by using extractive distillation in which the extractive agent is a higher boiling organic compound or a mixture of two or more of these. Typical examples of effective agents are: dimethylsulfoxide; dimethylsulfoxide and N,N-dimethylacetamide; dimethylsulfoxide, dimethylformamide and phthalic anhydride.

FIELD OF THE INVENTION

This invention relates to a method for separating t-amyl alcohol fromisobutanol using certain higher boiling liquids as the extractive agentin extractive distillation.

DESCRIPTION OF PRIOR ART

Extractive distillation is the method of separating close boilingcompounds from each other by carrying out the distillation in amultiplate rectification column in the presence of an added liquid orliquid mixture, said liquid(s) having a boiling point higher than thecompounds being separated. The extractive agent is introduced near thetop of the column and flows downward until it reaches the stillpot orreboiler. Its presence on each plate of the rectification columns altersthe relative volatility of the close boiling compounds in a direction tomake the separation on each plate greater and thus require either fewerplates to effect the same separation or make possible a greater degreeof separation with the same number of plates. The extractive agentshould boil higher than any of the close boiling liquids being separatedand not form minimum azeotropes with them. Usually the extractive agentis introduced a few plates from the top of the column to insure thatnone of the extractive agent is carried over with the lowest boilingcomponent. This usually requires that the extractive agent boil abouttwenty Centigrade degrees or more higher than the lowest boilingcomponent.

At the bottom of a continuous column, the less volatile components ofthe close boiling mixtures and the extractive agent are continuouslyremoved from the column. The usual methods of separation of these twocomponents are the use of another rectification column, cooling andphase separation, or solvent extraction.

Isobutanol and t-amyl alcohol are two of the most widely used alcoholsin commerce today. When they are used as solvents, they frequently endup as a mixture of solvents. Whenever practical, it is mandatory torecover the solvent and re-use it. t-Amyl alcohol boils at 102.4° C.,isobutanol at 108.1° C. and these two have a relative volatility of1.165, making it difficult to separate these two by rectification.Extractive distillation would be an attractive method of effecting theseparation of t-amyl alcohol from isobutanol if agents can be found that(1) will alter the relative volatility between t-amyl alcohol andisobutanol, (2) form no azeotropes with t-amyl alcohol or isobutanol and(3) are easy to recover from isobutanol, that is boil sufficiently aboveisobutanol to make the separation by rectification possible with only afew theoretical plates. Extractive distillation typically requires theaddition of an equal amount to twice as much extractive agent as theethanol-isopropanol on each plate of the rectification column. Theextractive agent should be heated to about the same temperature as theplate into which it is introduced. Thus extractive distillation imposesan additional heat requirement on the column as well as somewhat largerplates. However this is less than the increase occasioned by theadditional agents required in azeotropic distillation.

Another consideration in the selection of the extractive distillationagent is its recovery from the bottoms product. The usual method is byrectification in another column. In order to keep the cost of thisoperation to a minimum, an appreciable boiling point difference betweenthe compound being separated and the extractive agent is desirable. Werecommend twenty Centigrade degrees or more difference. It is alsodesirable that the extractive agent be miscible with the isopropanolotherwise it will form a two phase azeotrope with it and some othermethod of separation will have to be employed.

                  TABLE 1                                                         ______________________________________                                        Effect of Relative Volatility on the Separation of t-Amyl Alcohol             From Isobutanol at 99% Purity.                                                Relative                                                                             Theoretical                                                                             Actual Plates,                                                                             Actual Plates,                                  Volatility                                                                           Plates    75% Efficiency                                                                             75% Eff., Min. Reflux                           ______________________________________                                        1.165  60        80           104                                             1.4    27        36           47                                              1.6    20        27           35                                              1.8    16        22           28                                              2.0    13        18           24                                              2.2    12        16           21                                              ______________________________________                                    

The advantage of employing an effective extractive distillation agentfor this separation is shown in Table 1. When ordinary rectification isused, 104 actual plates of 75% efficiency are required at minimum refluxratio to separate t-amyl alcohol from isobutanol in 99% purity. Ifextractive distillation is employed with an agent that converts therelative volatility to 2.2, only 21 actual plates are required.

A number of investigators have reported the separation of lower boilingalcohols, one from another, by extractive distillation. Carlson, U.S.Pat. No. 2,570,205 used sulfolane to separate n-propanol from butylalcohols. Carlson, U.S. Pat. No. 2,551,584 used water in a steamdistillation on a mixture of lower alcohols but was unable to separatethe t-amyl alcohol from isobutanol. They remained in the same fraction.Drout, U.S. Pat. No. 2,552,412 used ethylene glycol, 1,3-propanediol anddiethylene glycol as the agents in separating a mixture containingethanol, sec. butanol, sec. amyl alcohol and t-amyl alcohols. Smith,U.S. Pat. No. 2,559,519 used glycol ethers to separate alcohol mixturescontaining ethanol, propanol, isopropanol, and sec. butanol. Smith, U.S.Pat. No. 2,559,520 used ethylene glycol and 1,3-butylene glycol as theextractive agent to separate n-propanol from sec. butanol. Carlson, U.S.Pat. No. 2,575,243 used glycol ether-esters as the agent to separaten-propanol from sec. butanol. Morrell, U.S. Pat. No. 2,591,712 usedparaffinic, naphthenic or aromatic hydrocarbon oils to separate closeboiling anhydrous alcohols. Morrell, U.S. Pat. No. 2,591,713 used waterand a white oil to separate the lower alcohols. Morrell, U.S. Pat. No.2,706,707 used aqueous solutions of sodium xylene sulfonate or sodiump-cymene sulfonate in the separation of lower alcohols.

OBJECTIVE OF THE INVENTION

The object of this invention is to provide a process or method ofextractive distillation that will enhance the relative volatility oft-amyl alcohol from isobutanol in their separation in a rectificationcolumn. It is a further objective of this invention to identify organiccompounds which are stable, can be separated from isobutanol byrectification with relatively few plates and can be recycled to theextractive distillation column and re-used with little decomposition.

SUMMARY OF THE INVENTION

The objects of this invention are provided by a process for separatingt-amyl alcohol from isobutanol which entails the use of certain organiccompounds as the agent in extractive distillation.

DETAILED DESCRIPTION OF THE INVENTION

We have discovered that dimethylsulfoxide (DMSO), either singly or inmixtures, will effectively enhance the relative volatility betweent-amyl alcohol and isobutanol and permit the separation of pure t-amylalcohol from isobutanol by rectification when employed as the agent inextractive distillation. Table 2 lists DMSO, its mixtures and theapproximate proportions that we have found to be effective. The data inTable 2 was obtained in a vapor-liquid equilibrium still. In each case,the starting material was a 50--50% t-amyl alcohol-isobutanol mixture.The ratios are the parts of extractive agent used per part of t-amylalcohol-isobutanol mixture. The relative volatilities are listed foreach of the two ratios employed. The compound that is effective whenused alone is DMSO. The compounds which are effective when used inmixtures of two or more components with DMSO are acetamide, benzoicacid, cinnamic acid, N,N-dimethylacetamide, ethylene glycol,1,4-butanediol, benzyl propionate, dipropylene glycol dibenzoate,phthalic anhydride, salicylic acid, trimellitic anhydride, tri-2-ethylhexyl trimellitate, dimethylformamide, hexahydrophthalic anhydride andethyl benzoate. The ratios in Table 2 are the parts of extractive agentused per part of t-amyl alcohol-isobutanol mixture. The two relativevolatilities correspond to the two different ratios. For example inTable 2, one part of DMSO with one part of the t-amyl alcohol-isobutanolmixture gives a relative volatility of 1.8, 6/5 parts of DMSO gives 2.1.One half part of DMSO mixed with one half part of N,N-dimethylacetamidewith one part of t-amyl alcohol-isobutanol mixture gives a relativevolatility of 1.5, 3/5 parts of DMSO plus 3/5 parts ofN,N-dimethylacetamide gives 1.6. One third parts of DMSO plus 1/3 partsof phthalic anhydride plus 1/3 parts of N,N-dimethyl-acetamide mixedwith one part of t-amyl alcohol-isobutanol mixture gives a relativevolatility of 1.8, with 2/5 parts, these three give 2.0. In everyexample in Table 2, the starting material is a 50--50% mixture of t-amylalcohol-isobutanol which possesses a relative volatility of 1.165.

                                      TABLE 2                                     __________________________________________________________________________    Extractive Distillation Agents Which Contain Dimethylsulfoxide                                                  Relative                                    Compounds                  Ratios Volatilities                                __________________________________________________________________________    None                       --     1.165                                       Dimethylsulfoxide (DMSO)   1  6/5.sup.                                                                          1.8                                                                              2.1                                      DMSO, Acetamide            (1/2).sup.2                                                                      (3/5).sup.2                                                                       1.5                                                                              1.5                                      DMSO, Benzoic acid         "  "   1.4                                                                              1.3                                      DMSO, Cinnamic acid        "  "   1.4                                                                              1.4                                      DMSO, N,N--Dimethylacetamide                                                                             "  "   1.5                                                                              1.6                                      DMSO, Ethylene glycol      "  "   1.5                                                                              1.4                                      DMSO, 1,4-Butanediol       "  "   1.6                                                                              1.4                                      DMSO, Benzyl propionate    "  "   1.4                                                                              1.4                                      DMSO, Dipropylene glycol dibenzoate                                                                      "  "   1.4                                                                              1.4                                      DMSO, Phthalic anhydride   "  "   1.5                                                                              1.4                                      DMSO, Salicylic acid       "  "   1.4                                                                              1.4                                      DMSO, Trimellitic anhydride                                                                              "  "   1.5                                                                              1.5                                      DMSO, Tri-2-ethyl hexyl trimellitate                                                                     "  "   1.4                                                                              1.4                                      DMSO, Dimethylformamide (DMFA), Acetamide                                                                (1/3).sup.3                                                                      (2/5).sup.3                                                                       1.6                                                                              1.5                                      DMSO, DMFA, Benzoic acid   "  "   1.4                                                                              1.4                                      DMSO, DMFA, N,N--Dimethylacetamide                                                                       "  "   1.5                                                                              1.5                                      DMSO, DMFA, Dipropylene glycol dibenzoate                                                                "  "   1.4                                                                              1.4                                      DMSO, DMFA, Phthalic anhydride                                                                           "  "   1.8                                                                              2.1                                      DMSO, DMFA, Hexahydro phthalic anhydride                                                                 "  "   1.4                                                                              1.5                                      DMSO, DMFA, Trimellitic anhydride                                                                        "  "   1.6                                                                              1.7                                      DMSO, DMFA, Tri-2-ethyl hexyl trimellitate                                                               "  "   1.5                                                                              1.4                                      DMSO, N,N--Dimethylacetamide (DMAC), Acetamide                                                           "  "   1.7                                                                              1.5                                      DMSO, DMAC, Ethyl benzoate "  "   1.5                                                                              1.5                                      DMSO, DMAC, Phthalic anhydride                                                                           "  "   1.8                                                                              2.0                                      DMSO, DMAC, Dipropylene glycol dibenzoate                                                                "  "   1.4                                                                              1.4                                      DMSO, DMAC, Salicylic acid "  "   1.4                                                                              1.4                                      DMSO, DMAC, Trimellitic anhydride                                                                        "  "   1.6                                                                              1.5                                      DMSO, DMAC, Cinnamic acid  "  "   1.4                                                                              1.4                                      DMSO, Acetamide, Trimellitic anhydride                                                                   "  "   1.4                                                                              1.4                                      __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Data From Run Made In Rectification Column.                                   Time     Stillpot Temp. °C.                                                               Overhead Temp.                                                                         Wt. % t-Amyl Alc.                                                                        Relative                               Agent Hrs.                                                                             At Start                                                                           Sampling                                                                           When Sampling                                                                          Overhead                                                                            Bottoms                                                                            Volatility                             __________________________________________________________________________    None  6       106  97.5     55.8  38.5  1.165                                 Dimethy-                                                                            0.5                                                                              106  118  97.5     63.3  41.9 1.21                                   sulfoxide                                                                     Dimethy-                                                                            1.5                                                                              106  121  97.5     81    46.3 1.43                                   sulfoxide                                                                     Dimethy-                                                                            2.5                                                                              106  139  97.5     78.6  21.4 1.78                                   sulfoxide                                                                     Dimethy-                                                                            3.5                                                                              106  167  97.5     88.9  33.3 1.85                                   sulfoxide                                                                     __________________________________________________________________________     Notes:                                                                        Agent feed rate: 20 ml/min; Agent temp.: 85° C.; Boilup rate: 10-2     ml/min.                                                                  

DMSO listed in Table 2 and whose relative volatility had been determinedin the vapor-liquid equilibrium still, was then evaluated in a glassperforated plate rectification column possessing 4.5 theoretical plates.The results are listed in Table 3. The t-amyl alcohol-isobutanol mixtureused contained about 45% t-amyl alcohol. The first run is with noextractive agent and with 220 grams of mixture in the stillpot. Aftersix hours of operation, the separation is that in accordance with arelative volatility of 1.165. The second run is with DMSO as theextractive agent. After one-half hour of continuous operation, therelative volatility was 1.21; after 1.5 hours, 1.43; after 2.5 hours,1.78 and after 3.5 hours, 1.85. Experience with this column has shownthat three or more hours of steady operation are required to reachequilibrium. The relative volatility attained, 1.85, can be comparedwith the 1.8 obtained for DMSO with the vapor-liquid equilibrium stilllisted in Table 2.

THE USEFULNESS OF THE INVENTION

The usefulness or utility of this invention can be demonstrated byreferring to the data presented in Tables 1, 2 and 3. All of thesuccessful extractive distillation agents show that t-amyl alcohol canbe removed from isobutanol by means of distillation in a rectificationcolumn and that the ease of separation as measured by relativevolatility is considerable. Without these extractive distillationagents, virtually no improvement will occur in the rectification column.The data also show that the most attractive agents will operate at aboilup rate low enough to make this a useful and efficient method ofrecovering high purity t-amyl alcohol from any mixture with isobutanol.The stability of the compounds used and the boiling point difference issuch that complete recovery and recycle is obtainable by a simpledistillation and the amount required for make-up is small.

WORKING EXAMPLES EXAMPLE 1

Twenty-five grams of t-amyl alcohol, 25 grams of isobutanol and fiftygrams of DMSO were charged to an Othmer type glass vapor-liquidequilibrium still and refluxed for 12 hours. Analysis of the vapor andliquid by gas chromatography gave a vapor composition of 53.1% t-amylalcohol, 46.9% isobutanol; a liquid composition of 38.6% t-amyl alcohol;61.4% isobutanol. This indicates a relative volatility of 1.80. Tengrams of DMSO were added and refluxing continued for another elevenhours. Analysis indicated a vapor composition of 47.5% t-amyl alcohol,52.5% isobutanol; a liquid composition of 30.1% t-amyl alcohol, 69.9%isobutanol which is a relative volatility of 2.10.

EXAMPLE 2

Thirty-three grams of t-amyl alcohol, 67 grams of isobutanol, 25 gramsof DMSO and 25 grams of N,N-dimethylacetamide were charged to thevapor-liquid equilibrium still and refluxed for five hours. Analysisindicated a vapor composition of 39.2% t-amyl alcohol, 60.8% isobutanol,a liquid composition of 29.8% t-amyl alcohol, 70.2% isobutanol which isa relative volatility of 1.53. Five grams of DMSO and five grams ofN,N-dimethylacetamide were added and refluxing continued for another sixhours. Analysis indicated a vapor composition of 38.5% t-amyl alcohol,61.5% isobutanol; a liquid composition of 29% t-amyl alcohol, 71%isobutanol which is a relative volatility of 1.53.

EXAMPLE 3

Thirty-three grams of t-amyl alcohol, 67 grams of isobutanol, 17 gramsof DMSO, 17 grams of N,N-dimethylacetamide and 17 grams of phthalicanhydride were charged to the vapor-liquid equilibrium still andrefluxed for 14 hours. Analysis indicated a vapor composition of 33.3%t-amyl alcohol, 66.7% isobutanol; a liquid composition of 21.6% t-amylalcohol, 78.4% isobutanol which is a relative volatility of 1.82. Threegrams each of DMSO, N,N-dimethylacetamide and phthalic anhydride wereadded and refluxing continued for another ten hours. Analysis indicateda vapor composition of 36.8% of t-amyl alcohol, 63.2% isobutanol and aliquid composition of 22.7% t-amyl alcohol, 77.3% isobutanol which is arelative volatility of 1.98.

EXAMPLE 4

A glass perforated plate rectification column was calibrated withethylbenzene and p-xylene which possesses a relative volatility of 1.06and found to have 4.5 theoretical plates. A solution of 100 grams oft-amyl alcohol and 100 grams of isobutanol was placed in the stillpotand heated. The column was operated at total reflux for six hours toestablish equilibrium throughout. Overhead and bottoms samples ofapproximately 2 ml. were collected and analysed by gas chromatography.The overhead analysis was 55.8% t-amyl alcohol, 44.2% isobutanol and thestillpot analysis was 38.5% t-amyl alcohol and 61.5% isobutanol. Usingthese compositions in the Fenske equation with the number of theoreticalplates in the column being 4.5, gave an average relative volatility of1.165 for each theoretical plate. An extractive agent consisting of DMSOwas then pumped into the column at a rate of 20 ml/min. The temperatureof the extractive agent as it entered the column was 85° C. Afterestablishing the feed rate of the extractive agent, the heat input tothe t-amyl alcohol-isobutanol in the stillpot was adjusted to give areflux rate of 10-20 ml/min. After one half hour of operation, overheadand bottoms samples were collected and analysed. The overhead analysiswas 63.3% t-amyl alcohol, 36.7% isobutanol and the bottoms analysis was41.9% t-amyl alcohol, 58.1% isobutanol which gave an average relativevolatility of 1.21 for each theoretical plate. After 1.5 hours of totaloperating time, the overhead and bottoms samples were again taken andanalysed. The overhead composition was 81% t-amyl alcohol, 19%isobutanol and the bottoms composition was 46.3% t-amyl alcohol, 53.7%isobutanol. This gave an average relative volatility of 1.43 for eachtheoretical plate. After 2.5 hours of total operating time, the overheadand bottoms samples were again taken and analysed. The overheadcomposition was 78.6% t-amyl alcohol, 21.4% isobutanol and the bottomscomposition was 21.4% t-amyl alcohol, 78.6% isobutanol. This gave anaverage relative volatility of 1.78 for each theoretical plate. After3.5 hours of total operating time, the overhead analysed 88.9% t-amylalcohol, 11.1% isobutanol and the bottoms analysed 33.3% t-amyl alcohol,66.7% isobutanol which is a relative volatility of 1.85 for eachtheoretical plate. The data in this example are summarized in Table 3.

We claim:
 1. A method for recovering t-amyl alcohol from a mixture oft-amyl alcohol and isobutanol which comprises distilling a mixture oft-amyl alcohol and isobutanol in a rectification column in the presenceof about one to two parts of extractive agent per part of t-amylalcohol-isobutanol mixture, recovering the t-amyl alcohol as overheadproduct and obtaining the extractive agent and isobutanol from thestillpot, the extractive agent comprises dimethylsulfoxide.
 2. Themethod of claim 1 in which the extractive agent comprisesdimethylsulfoxide and at least one material from the group consisting ofacetamide, benzoic acid, cinnamic acid, N,N-dimethylacetamide, ethyleneglycol, 1,4-butanediol, benzyl propionate, dipropylene glycoldibenzoate, phthalic anhydride, salicylic acid, trimellitic anhydride,tri-2-ethyl hexyl trimellitate, dimethylformamide, hexahydrophthalicanhydride and ethylbenzoate.